The regulated assembly of actin is critical for cell motility, maintenance of shape and adhesion. A variety of extracellular stimuli induce reorganization of the actin cytoskeleton, but the detailed mechanisms by which extracellular stimuli lead to actin reorganization remain unclear. The long-term objective of this proposal is to define the molecular basis of regulated actin assembly in cells; in this proposal, we specifically address the role of actin filament fragmentation in stimulus induced shape change. Actin filament severing and capping proteins have been postulated to be key regulators of actin dynamics in the cell by fragmenting filaments and increasing the number of filament ends. In vitro, an increase in the number of regulated high affinity ends increases the conversion of monomer to filament, while increasing the number of low affinity ends increases the conversion of filaments to monomer. Using fluorescent analogs and specific probes we will test the hypothesis that actin filament severing is a consequence of stimuli which induce specific types of shape change and motility.